It all depends on your own ability to nuke and survivability.

For example, if you are low on healers or you have no healer. Your main concern is incapacitating their DPS.

If you have sufficient burst, you kill DPS first. If you have CC but not enough burst. You cc the DPS (Just try your best to avoid getting damage through any means) as you kill the healer. The very start of the fight. (Not start of arena, but the very moment you or they engage in the arena). Also can mean a change of strategy is required. Sometimes you would find yourself into a position to momentarily move any one char out of sight of any other char. These mistakes are rarely made on high rated but can be regularly seen on low-average rating. If you play 3 dk's. I think a good 'start' position would be behind or inbetween any objects. Lure them to one side of the object by posing on that side of the object and be prepared to pull and move out of line of sight if they're ranged and have burst dps, for example. But I'm guessing now, never played 3 dk's. It's up to you to find strats that work for you best.

The key to success in arena is quite simple. (The execution is the hard part.)

1. Patience. Never rush a game. And try to learn from every fight. (Not what you did wrong, but what you could have done to prevent it.)
2. Quantity over quality. Play games, many games, many many games. The more games you play, the more experience you will get. If that means you will drop all the way to 1200 at first, don't bother your head about it. If you only play 10 games a week, chances are you won't improve quickly. Play a 100 a week and you will most definatly see improvement.

When I did 5v5's in TBC (quad shammy), I pretty much sucked at the beginning. First week, 1 win 9 losses. I didn't feel like continueing and stopped. Second week it started the same, but I was like, argh, I can do this. Kept playing games and more games. Eventually I ended that week with approximatly 70 losses and 40 wins. But the last 40 games had like a 80% win chance and got me back to 1600 rating in my second week. Anyhow, I kept that up for a few weeks. Just playing, studying, learning. I haven't stopped climbing since. My aim was always to end at least 50 rating above my last weeks rating. I was still gearing up so didn't want to aim too high. Some weeks I easily got 100 rating more. I was 1900+ rating when Patch 3.0 started and screwed up arena, but the very last week before the patch I still climbed 85 rating. I've no doubt I would've reached 2k+ rating soon enough had the patch come a little later. :P What I'm saying is. Don't give up. Working out strategies and learning to play your characters in dynamic situations takes time. Find out why other teams beat you and devise strategies that would prevent them from doing just that. Work on perfecting those strategies and I'm sure you will be able to see success.

I'm not sure how high 3 dk's can potentially go. But I've no doubt you should be able to hold your own at 1500's/1600's, which is respectable enough as a multiboxer. :P